Method for making crystallized fructose

ABSTRACT

The invention relates to a method for making crystallized fructose comprising preparing a pure fructose syrup by melting fructose dihydrate crystals, concentrating the melt by heating under reduced pressure to obtain a dry matter content above 96% by weight, seeding the concentrated syrup with fructose crystals, and solidifying the seeded syrup. In a preferred process, the time between the melting the fructose dihydrate crystals and the concentration of the syrup to a dry matter content above 96% by weight is less than 24 hours, preferably less than 8 hours. The invention also relates to crystallized fructose produced by the process of the present invention wherein the fructose is comprised primarily of the beta-D-fructopyranose fructose crystals tautomers with low amounts of other fructose tautomers which are considered impurities in the crystallization context.

This application is a 371 of PCT/EP99/09732 filed Jul. 25, 1999.

The present invention addresses a process for manufacturing crystallinefructose by crystallizing/solidifying a purified and concentratedfructose syrup.

The preparation of crystalline fructose from sugar syrups, particularlysyrups containing both fructose and glucose is known per se but involvesa number of practical problems.

U.S. Pat. No. 2,588,449 discloses a process for preparing crystals offructose dihydrate C₆H₁₂O₆.2H₂O, by cooling and seeding a fructosesolution. However, the product obtained thereby, fructose dihydrate, isonly stable at relatively low temperatures and can be neither stored nortransported without particular precautions to prevent liquefactionthereof.

On the other hand the so-called “anhydrous” fructose (see U.S. Pat. No.3,513,023) is known. The methods for crystallizing this product areeither relatively complex and costly, e.g. using alcoholic solutions(see U.S. Pat. No. 3,607,392), or appear to be ineffective in practicedue to isomerization of fructose in the aqueous phase. Particularly,this is the case of the process according to U.S. Pat. No. 3,513,023:when the conditions claimed by this patent are recreated experimentally,it happens that fructose “mutarotation” (equilibrium reaction betweenthe five isomers of fructose, i.e. beta-D-fructopyranose,beta-D-fructofuranose, alpha-D-fructofuranose, alpha-D-fructopyranoseand the ketonic, noncyclic form of fructose) causes the formation ofnoncrystallizable isomers, which considerably affects crystallization bysolidification.

The present invention has the object to provide a new fructosecrystallization process while obviating the problems of prior art and toobtain a form of commercially useable crystalline fructose.

SUMMARY OF THE INVENTION

The present invention relates to a method for making crystallizedfructose comprising preparing a pure fructose syrup by melting fructosedihydrate crystals, concentrating the melt by heating under reducedpressure to obtain a dry matter content above 96% by weight, seeding theconcentrated syrup with fructose crystals, and solidifying the seededsyrup. In a preferred process, the time between the melting the fructosedihydrate crystals and the concentration of the syrup to a dry mattercontent above 96% by weight is less than 24 hours, preferably less than8 hours. The invention also relates to crystallized fructose produced bythe process of the present invention wherein the fructose is comprisedprimarily of the beta-D-fructopyranose fructose crystals tautomers withlow amounts of other fructose tautomers which are considered impuritiesin the crystallization context.

DETAILED DESCRIPTION OF THE INVENTION

This new process for manufacturing crystallized fructose, includingsolidification of a hot and concentrated fructose solution after seedingthe solution with fructose crystals is characterized by the use of afructose solution resulting from a pure fructose syrup, freshly preparedby melting crystals of fructose dihydrate, and concentrated byevaporation in a vacuum to obtain a dry matter content above 96% byweight, and preferably above 97% by weight.

In accordance with an additional characteristic of the invention, thefructose solution used for seeding preferably has a temperature of 50 to100° C., ideally of 80 to 95° C.

Also, seeding is preferably performed with 5 to 30% by weight of finelygrained crystallized fructose, particularly having a particle size ofless than 500 μm, preferably less than 250 μm (for instance 50 μm).

According to another additional characteristic of the invention, thetime between melting of fructose dihydrate crystals to prepare thestarting syrup and concentration of said syrup to a dry matter contentabove 96% by weight, preferably above 97% by weight is preferably ofless than 24 hours, more particularly less than 8 hours, and ideallyless than half an hour.

The invention also relates to a general process for manufacturingcrystallized fructose from a syrup containing fructose and possiblyother substances (for instance other sugars, proteins, etc.), includingthe following successive steps:

seeding, with crystals of fructose dihydrate, the aqueous solution offructose having a total sugar content of 45 to 65% by weight and afructose content above 60% by weight, referred to the dry matter, at atemperature of 10 to −10° C.,

keeping the solution in fructose supersaturation conditions, byprogressively lowering temperature until crystals of the desired sizeare obtained, and

separating the crystals obtained from the mother liquor to obtaincrystals of fructose dihydrate having a fructose content of 95 to 100%by weight referred to the dry matter;

melting the crystals collected in a syrup having a content of about 83%by weight of sugar and about 17% of water;

concentrating the syrup obtained thereby by water evaporation underlimited pressure, preferably a pressure of less than a column of mercury60 mm high, until a dry matter content above 96% by weight, preferablyof 97 to 100% by weight is reached;

seeding the syrup obtained thereby by means of 5 to 30% by weight ofcrystallized fructose, preferably at a temperature of 80 to 95° C.;

keeping the mass obtained thereby at a temperature of 55 to 75° C.,until a nontacky and brittle mass is obtained, and

crushing, drying and grinding this mass into particles formed bycrystals of agglomerated fructose.

It will be understood that a possible water addition upon melting ofcrystals of fructose dihydrate, and the consecutive removal of saidadded water upon concentration of the syrup until a dry matter contentof 96 to 100% by weight is reached are absolutely equivalent to themethod which provides melting of crystals as collected, provided thatthe time between syrup formation and concentration thereof is as shortas possible (particularly below 24 hours).

The invention also relates, as a new product, to crystallized fructose,essentially consisting of beta-D-fructopyranose, having a poor contentof other fructose tautomers, resulting from a method of solidificationof a hot and concentrated fructose solution, after seeding the solutionwith crystals of fructose, in which the fructose solution used forseeding results from a pure fructose syrop freshly prepared by meltingcrystals of fructose dihydrate, concentrated by evaporation in a vacuumto a dry matter content of 96 to 100% by weight,

the fructose solution used for seeding preferably has a temperature of80 to 95° C. and seeding is performed by using 5 to 30% by weight ofcrystallized fructose having a particle size of less than 500 μm,preferably of less than 250 μm, and

the time elapsing from melting of fructose dihydrate crystals to preparethe starting syrup to concentration of said syrup to a dry mattercontent above 96% is of less than 8 hours, preferably less than half anhour.

The following examples will be useful to disclose the invention andspecify a few preferred details and embodiments, without limitingthereby the scope as defined in the claims below.

EXAMPLE I

Purification of a glucose and fructose solution: obtaining crystals offructose dihydrate C₆H₁₂O₆.2H₂O

Crystals of fructose dihydrate are obtained by progressively cooling anaqueous solution of glucose and fructose whose total sugar content is of45 to 85% and in dry matter, fructose concentration is of 60 to 100%.

During the cooling process, when the saturation concentration offructose dihydrate is obtained, crystals of fructose dihydrate are addedin the mass of the syrup from the outside. This addition of externalcrystals, named seeding, takes place at a temperature ranging from 10 to−10° C. depending on dry matter concentration and on dry mattercomposition.

After seeding, the temperature of the mass so seeded, is progressivelylowered and thereafter kept in such fructose supersaturation conditionsthat the existing crystals increase progressively whereas the appearanceof new crystals is limited.

Once the crystals have reached the desired size, the syrup masscontaining the crystals is conveyed towards a continuous ordiscontinuous centrifuge, similar to those used in sugar production. Anyother physical solid/liquid separation method may be provided.

Crystallization mother liquor, which consists of any noncrystallizedwater and sugars of the starting solution may be submitted to a newcrystallization stage as described above.

The fructose dihydrate crystals, thereby separated, may be washed formaximum removal of the mother liquor film which surrounds them therebyimproving fructose purity to values of 90% to 100%, preferably of 97.5%to 100%. Then, they are melted, preferably at temperatures above 20° C.If no water is added during melting, the syrup obtained from properlywashed crystals contains 83% of sugars and 17% of water. The dry matterof this syrup only contains a few traces of the other mother liquorsugars.

EXAMPLE II

Concentration of a syrup obtained by melting crystals of fructosedihydrate.

The syrup has a 83% of dry matter obtained according to the example 1and concentrated at temperatures of the order of 65 to 100° C. underpressures of less than a 60 mm high column of mercury, for a time ofless than 30 minutes, or less than 15 minutes. The required maximumtemperature decreases with decreasing concentration pressure, therebyreducing the risk of discoloration of the concentrated syrup.

Depending on temperature, pressure and concentration time, the drymatter of the concentrated syrup varies from 96.0 and 100.0%. At anoutlet temperature of the order of 90° C., the syrup is relativelyviscous and may be conveyed, i.e. pumped to be subsequentlycrystallized/solidified.

Note:

In fructose dihydrate crystals, as well as in fructose crystals, thefructose molecule only has the form of beta-D-fructopyranose.Conversely, in the aqueous solution, fructose is subject to amutarotation phenomenon. In equilibrium, the solution contains thefollowing 5 isomers:

beta-D-fructopyranose, beta-D-fructofuranose, alpha-D-fructofuranose,alpha-D-fructopyranose and the ketonic noncyclic form of fructose (thelatter 2 forms being much minor portions). Only the first of the aboveisomers may enter the crystal texture of fructose, whereas the othersshall have to be considered as impurities in the crystallizationcontext. In order to prevent as far as possible the formation of thesenoncrystallizable isomers, syrups (obtained by melting dihydratecrystals) should be rapidly concentrated, to a dry matter contentpreferably above 96.0%, ideally to a dry matter content above 97.0%.Mutarotation takes place as slowly as the water content in the solutionis low. If the time from crystal melting to syrup concentration isexcessively long, the noncrystallizable isomer content may reach valuesof the order of 25 to 30% and this affects crystallization ofbeta-D-fructopyranose to a considerable extent.

EXAMPLE III

Crystallization/solidification of a fresh concentrated syrup.

The concentrated and viscous syrup obtained from Example II is kept in ablender at a temperature of 80 to 95° C. and seeded with 10 to 25% ofvery finely grained crystallized fructose (<200 μm). Seeding may beperformed by using the fructose obtained from the previous process,after grinding it finely. Then, the medium is stirred to ensure optimaldispersion of nuclei therein. The medium consisting of a highly viscouswhitish mass is poured out of the blender and stored in an oven whosetemperature is set at a value of 55 to 75° C. depending on the residualwater content, on the number of nuclei and on the mutarotation progress.The time of permanence in the oven also depends on these factors.

At this stage, crystallization occurs until the whole eventually forms anontacky, rigid and brittle mass.

It has to be noted that, if a non seeded syrup is treated in the samemanner, it will completely vitrify and form an amorphous(noncrystalline) translucent and highly hygroscopic mass which cannot beneither ground nor stored.

The crystallized mass obtained thereby is coarsely ground by means of agrinder. Then, the particles so obtained may be dried at a temperatureof 50 to 75° C. (preferably under dry air flow) to further reduce thewater content. Then a fine grinding may be performed. If the initialmass is dry enough to be finely ground, the drying step following coarsegrinding may be skipped.

The product obtained thereby is composed of particles (whose particlesize depends on the fine grinding step), which in turn consist of smallagglomerated fructose crystals. In order to remove as much water aspossible, a final drying step, at a temperature of 50 to 75° C. takesplace at the end of the fine grinding step.

What is claimed is:
 1. A process for manufacturing crystallized fructosecomprising 1) preparing a pure fructose syrup by melting fructosedihydrate crystals, 2) concentrating the melt by heating under reducedpressure to obtain a dry matter content above 96% by weight, 3) seedingthe concentrated syrup with fructose crystals, and 4) solidifying theseeded syrup.
 2. The process according to claim 1 wherein theconcentrated syrup is seeded at a temperature of 50 to 100° C.
 3. Theprocess according to claim 1 wherein the seeding is performed with 5 to30% by weight of fructose crystals having a particle size of less than500 μm.
 4. The process according to claim 1 wherein the time between themelting the fructose dihydrate crystals and the concentration of thesyrup to a dry matter content above 96% by weight is less than 24 hours.5. The process according to claim 1 wherein the time between the meltingof the fructose dihydrate crystals and the concentration of the syrup toa dry matter content above 96% by weight is less than 8 hours.
 6. Theprocess according to claim 1 wherein the time between the melting offructose dihydrate crystals and the concentration of the syrup to a drymatter content above 96% by weight is less than half an hour.
 7. Theprocess according to claim 1 wherein the heating is performed in avacuum.
 8. A process for manufacturing crystallized fructose comprising:preparing a solution of fructose having a total sugar content of 45 to85% by weight and a fructose content above 60% by weight, referred tothe dry matter and seeding the solution with fructose dihydrate crystalsat a temperature between 10 to −10° C., progressively lowering thetemperature of the seeded solution until fructose dihydrate crystalshaving a fructose content of 95 to 100% by weight of dry matter areobtained, and separating the crystals from the seeded solution; meltingthe resulting crystals to obtain a syrup comprised of about 83% byweight sugar and about 17% water; concentrating the syrup by evaporatingthe water under reduced pressure to obtain a dry matter content of 96%to 100% by weight; seeding the concentrated syrup with 5 to 30% byweight of crystallized fructose, keeping the resulting mass at atemperature between 55 to 75° C. until a nontacky and brittle mass isobtained, and crushing, drying and grinding the nontacky brittle massinto particles of agglomerated fructose crystals.
 9. The processaccording to claim 8 wherein the temperature of the concentrated syrupis between 80 to 95° C.
 10. The process according to claim 9 wherein thesyrup is concentrated by evaporating the water in a vacuum.
 11. Theprocess according to claim 10 wherein the syrup is concentrated to a drymatter content above 96% by weight and the crystals used to seed theconcentrated syrup have a particle size of less than 250 μm.
 12. Aprocess of producing crystallized beta-D-fructopyranose comprising:preparing a fructose syrup by melting fructose dihydrate crystals andconcentrating the syrup by heating in a vacuum to a dry matter contentabove 97% by weight, seeding the concentrated syrup at a temperature of80 to 95° C. with 5 to 30% by weight of crystallized anhydrous fructosehaving a particle size of less than 500 μm, wherein the time between themelting of the fructose dihydrate crystals and the concentration of thesyrup to a dry matter content above 97% by weight is less than half anhour.
 13. Crystallized beta-D-fructopyranose prepared by the process ofclaim 12.